A model for predicting EEOAE output under acoustical modulation. (a and b) Schematic basal cross-sections through the high-frequency area of the auditory papilla of Tiliqua rugosa to illustrate the influence of hair-cell orientation on EEOAE output for two possible locations for the motor driving the cochlear amplifier. The tallest stereovilli of the hair-cell groups on either side face the middle of the papilla. A hair-cell lateral membrane motor would drive all cells to elongate and shorten in phase (a, arrows) and EEOAE from different populations would add constructively. For a bundle-based motor, however, oppositely oriented bundles would move 180° out-of-phase (b, arrows) and EEOAE add destructively. tm, tectorial membrane; sc, supporting cells; hc, hair cell, hcb, hair-cell stereovillar bundle. When low-frequency sound is applied, the output from hair cells of one orientation decreases, whereas that of the other increases, the force from each type being of the same phase in the case shown in a and c--e and of opposite phase in the case b and f--h. A 5% excess of cells of one polarity is assumed, allowing a small EEOAE response at the resting point in f–h. The model uses the same frequency ratio between electrical and acoustical stimuli as in a–c in Fig. 4 (9.25:1). The time axes cover just over 1.5 cycles of the modulator. (c—e) EEOAE output (thin lines) of a model simulating lateral membrane motors stimulated by ac current and modulated by a low-frequency sound (thick line, placed on arbitrary axes relative to the EEOAE). Compared with c, the relative sound pressure of the modulator is increased by 10 and 20 dB in d and e, respectively. (f—h) As in c and d, but for motors in the hair cell bundle. Modulation increases EEOAE from one set of cells and reduces it from the other, depending on the phase of the modulating sound. Compared with f, the sound pressure of the modulator is increased by 10 and 20 dB in g and h, respectively. In e and h, the vertical lines emphasize the roughly 180° phase shift in the EEOAE in h at each half cycle of the modulator.

EEOAE waveforms in the bobtail lizard measured during electrical stimulation and modulation by low-frequency tones. (a—c) EEOAE generated in Scala media by ac current (5.0 μA) of frequency 1593 Hz, modulated by a tone of 172 Hz with levels of 68, 78, and 88 dB SPL, respectively. (d—f) EEOAE from a different animal, in this case generated by ac current (3.8 μA) of frequency 2497 Hz, modulated by 344 Hz tones at 86, 96, and 106 dB SPL, respectively.

The spectrum of EEOAE (measured in steps of 172 Hz) produced by injecting 4.5 μA ac through a micropipette into Scala media of one ear of the Bobtail lizard (continuous line). The spectrum measured during current injection but when the bias voltage to the condensor microphone capsule was turned off gives the electrical noise floor of the recording system and is shown by the dashed line.

Modulation of the EEOAE in the bobtail lizard by low-frequency sound. (a) An EEOAE waveform produced by injecting 3.5 μA of current at 1830 Hz. (b) The waveform measured under the same current conditions but with the microphone bias turned off. (c) As in a, but with a tone added at 86 Hz and 95 dB SPL. The time scale covers four cycles of the modulator waveform. (d) The amplitude of the EEOAE signal (●), the lower intermodulation side band (▿), and the upper side band (▵) as a function of the modulator level.

Physiological vulnerability of EEOAE in the bobtail lizard. The effect in one ear of diffusion of streptomycin from the electrode is shown as a function of elapsed time after placing the electrode in Scala media. Each panel in a–d covers four cycles of the modulator waveform. (a) Control measurement taken with a KCl electrode before streptomycin. (b) Waveform after 5 min of streptomycin in Scala media. (c) Waveform after 29 min. (d) Fall in the EEOAE level (●) as a function of time after placement of the streptomycin electrode at time 0. In the first hour, there is a great loss in the amplitude of the lower (▿) and upper (▵) side bands of the modulated signal.